The advent of low voltage digital circuitry in present day electronics resulted in the use of many power converters which, for example, take the input power supply, ex. battery power, and down convert the battery voltage to a voltage suitable for operation of the electronic circuitry. Switch mode power supplies are needed and used in many electronic devices as they are more efficient in power conversion than linear regulator power supplies. Switch mode power supplies can be either magnetic based or capacitor based.
In a regular magnetic based buck converter, one terminal of the inductor is connected to the output and the other terminal is switched between the voltages equal to the positive supply (input voltage) and negative supply (generally, ground) with the time duration of the positive supply as a fraction of one switching period being represented by a duty cycle.
Magnetics based converters are efficient but need magnetic components like Inductors and transformers which are bulky. In addition to making electronic devices bigger, the bulky magnetic components, due to their larger height, have a further disadvantage that they cannot be co-packaged with the components they are powering. As the converters are placed further away from the components they are powering, the increased parasitic trace resistances and capacitances further decrease the conversion efficiency and bandwidth. Furthermore, as the current in a magnetic component cannot be changed instantaneously, the presence of magnetic components limits the loop bandwidth of the converter.
Another conversion approach is the use of a switched capacitor based converter. FIG. 1 shows, generally at 100, a 2:1 step-down converter. It consists of two capacitors C1 and C2 and four switches S1, S2, S3 and S4. These four switches are operated by two anti phase clocks Phi1 and Phi2. When Phi1 is high, switches S1 and S3 are ON and switches S2 and S4 are OFF, thus connecting the capacitors C1 and C2 in series with the input VIN and ground. Therefore, the sum of the voltages on these capacitors must be equal to the input voltage VIN. When Phi2 is high, switches S2 and S4 are turned ON and switches S1 and S3 are OFF, thus connecting the capacitors C1 and C2 in parallel with each other and in turn parallel with the output VOUT. This equalizes the voltages on both the capacitors. Since the sum of the voltages on the capacitors is equal to the input voltage VIN and in addition, these two capacitors carry equal voltages, the voltage across each capacitor is therefore equal to half of the input voltage VIN.
Capacitors have a ripple voltage around their steady state value of VIN/2. When Phi2 is high, capacitor C1 is discharging and capacitor C2 is charging and when Phi1 is high, C1 is charging from VIN and C2 is discharging by a load at VOUT. When Phi1 goes high, C1 is instantaneously charged such that sum of the voltages on C1 and C2 are equal to VIN. The charge current and charge speed are only limited by the switch resistances. Similarly, when Phi2 goes high, C2 is charged instantly such that voltages on C1 and C2 are made equal. Every switching cycle energy equal to ½*C*(ΔV)2, where C is equal to the capacitance value and ΔV is the capacitor voltage ripple, is lost as heat. Furthermore, since capacitor voltages are regulated by a combination of putting them in parallel and then putting them in series with the input, only discrete voltage conversion ratios are possible. For example, with two capacitors, each capacitor can only be regulated to VIN/2; therefore, if a voltage other than VIN/2 is required, then it is not possible with two capacitors. This type of converter does not need the bulky magnetic components like inductors but since only discrete voltage conversion ratios are possible, switched capacitor based converters are only efficient when the desired output voltage is close to the discrete voltage conversion ratio possible with the converter.
Thus there is a need for a power supply which is compact with decreased magnetic component size, efficient, has a better and more efficient capacitor voltage regulation, which offers wider bandwidth without impacting conversion efficiency and which is integration friendly with the components it is powering. This presents a technical problem for which a technical solution using a technical means is needed.